When two objects collide, they transfer momentum and energy between each other. Momentum, the product of an object’s mass and velocity, is the primary quantity exchanged, and it shifts from one object to the other through the forces they exert during contact. Energy also transfers, though some of it changes form in the process.
Momentum: The Main Thing That Transfers
Momentum is the central quantity that moves between colliding objects. It’s calculated by multiplying an object’s mass by its velocity, and it has both a size and a direction. When a moving billiard ball strikes a stationary one, the first ball slows down and the second speeds up. What happened is a transfer of momentum from the first ball to the second.
The reason momentum is so important in collisions is the law of conservation of momentum: in any collision where outside forces (like friction with the ground) are small enough to ignore, the total momentum before and after the collision stays exactly the same. Momentum isn’t created or destroyed. It only shifts between the objects involved. If a heavy truck rear-ends a small car, the truck loses some momentum and the car gains that same amount. The total for the system doesn’t change.
This conservation law works as a vector equation, meaning direction matters. A ball bouncing off a wall reverses its direction, so the wall absorbs momentum in one direction and pushes it back in the other. This is why pool players can predict exactly where balls will travel after a shot: the momentum math is precise and reliable.
How Impulse Delivers the Transfer
The mechanism that actually moves momentum from one object to another is called impulse. Impulse equals the force between the two objects multiplied by the time they’re in contact. A large force over a short time can deliver the same impulse as a smaller force over a longer time.
This relationship is why the duration of a collision matters so much. If a collision brings a moving object to a stop, the total change in momentum is fixed. Stretching that collision over a longer time interval reduces the peak force. This is the entire principle behind catching a baseball with soft hands rather than stiff ones, or bending your knees when you land from a jump. The momentum transfer is the same either way, but the force your body experiences drops dramatically when you extend the contact time.
Kinetic Energy Transfers Too, but Differently
Energy also transfers between colliding objects, but unlike momentum, kinetic energy (the energy of motion) is not always conserved. What happens to it depends on the type of collision.
In a perfectly elastic collision, kinetic energy transfers between the objects without any being lost. Both momentum and kinetic energy are fully conserved. Think of a Newton’s cradle: when one steel ball swings in and strikes the row, the ball on the opposite end swings out with nearly the same speed. The kinetic energy passed through the chain almost perfectly. Billiard ball collisions come close to this ideal as well.
How much kinetic energy transfers depends on the mass ratio between the two objects. When two objects have equal mass and one is initially stationary, the moving object can transfer virtually all of its kinetic energy to the other. As the masses become more unequal, the maximum possible transfer drops. A ping-pong ball hitting a bowling ball transfers almost none of its energy, bouncing back with nearly the same speed it arrived with.
Where “Lost” Energy Actually Goes
Most real collisions are inelastic, meaning some kinetic energy is converted into other forms during the impact. The total energy in the system is still conserved (energy can’t be destroyed), but it shifts into forms that no longer propel the objects forward.
The biggest destination for that energy is heat. When two cars collide, the metal crumpling and grinding generates thermal energy in the deformed material. You can feel this yourself: clap your hands hard and your palms warm up. That warmth is kinetic energy that converted to heat during the impact.
Sound is another energy sink. The bang you hear when two objects crash together is acoustic energy radiating outward, energy that used to be kinetic. In a violent collision like a car crash, the sound alone can carry a noticeable fraction of the original energy. Permanent deformation of the objects absorbs energy too. Bending, denting, or breaking an object requires energy, and that energy comes directly from the kinetic energy the objects had before the collision. If a material deforms within its elastic limits (like a rubber ball), most of that energy bounces back. If the material yields permanently (like a crumpled fender), that energy is gone from the motion of the system for good.
Why This Matters in Car Safety
Engineers use these principles to save lives. Crumple zones, the collapsible structures at the front and rear of modern cars, are designed to absorb kinetic energy through progressive deformation so that energy doesn’t transfer to the passenger compartment. The front and rear crumple zones of a typical car are engineered to collapse at a force that limits horizontal deceleration to about 20g for the rigid passenger cage inside.
Before crumple zones, the kinetic energy of a crash transferred much more directly to the occupants. Older, stiffer car frames experienced shorter collision times, which meant higher forces (the impulse relationship at work again). Modern designs intentionally extend the deformation process, lowering the peak force on passengers while converting kinetic energy into the work of bending metal, generating heat, and producing sound. The choice of materials and geometry in these zones has been one of the most consequential safety innovations in automotive history.
Putting It All Together
In every collision, momentum transfers between objects and is conserved overall. Kinetic energy also transfers, but some portion typically converts into heat, sound, and permanent deformation. The force experienced during the transfer depends on how long the objects are in contact: longer contact means lower force for the same momentum change. These aren’t separate phenomena. They’re all connected through Newton’s laws, and they explain everything from why a cue ball stops dead on a perfect shot to why modern cars are designed to crumple rather than stay rigid.